Misfiring of internal combustion engines is an operational disturbance which implies that the fuel/air mixture compressed in the cylinder is not completely burnt, or not burnt at all, for example because of a failing ignition spark (Otto engine) or disturbances during injection (Diesel engine). It therefore results in reduced power of the internal combustion engine, increased wear on components included and, in addition, entails increased emission of environmentally harmful substances as, for example, unburnt hydrocarbon compounds. Misfiring also contributes to reduction of the service life of the exhaust gas catalytic converter of the internal combustion engine. Detecting and indicating misfiring is, therefore, of great importance for an increased service life of internal combustion engines. Detection also permits a possibility of reducing the environmental influence and of complying in a better way with the increasingly stricter demands made by the legislation on emission of various pollutants from internal combustion engines.
One way of defining misfiring is based on the work which is carried out in each cylinder during a work cycle, which, in the case of a conventional four-stroke Otto engine or diesel engine, consists of two crankshaft turns (720.degree.). If the work for a cylinder is insignificant in relation to the amount of fuel present in the cylinder, it can be said that, by definition, misfiring has taken place. As a measure of this work, a so-called indicated mean effective pressure (IMEP) is often used, which is obtained by dividing the measured work by the piston displacement of the cylinder.
Misfiring can be detected in a plurality of different ways.
A direct measurement of the cylinder pressure in each individual cylinder may, of course, be used for detecting misfiring. Because of the severe environment in which the necessary pressure sensors are to operate, the method is costly and has substantially been used only in very large internal combustion engines. Since the method is based on a comparison of the output signals from several different sensors, varying aging between the sensors may lead to problems with the reliability of the method after a long time. In addition, it is necessary to have access to the crank angle for each cylinder with great accuracy in order for the calculation of the work carried out to become reliable.
Another method for detecting misfiring comprises measuring the electrical conductivity of the gas present in the cylinders of the internal combustion engine. This measurement may be performed by using the spark plug of the respective cylinder as measuring electrode. Such a method is described, for example, in U.S. Pat. No. 5,207,200. During a misfire, the gas in the cylinder becomes colder than after a successful ignition, and the gas therefore has a lower conductivity during a misfire. An advantage of this method is that an electrical quantity may be measured directly without needing access to a separate sensor. However, the method places special demands on the design of the ignition system of the internal combustion engine. As an example, it may be mentioned that special spark plugs are required as well as a separate ignition coil for each cylinder. Problems may arise due to varying aging of the spark plugs and the reliability during no-load operation, acceleration and braking is not good. This, method makes demands for a very high calculation capacity. Additional methods for detecting misfiring are based on measurement of the engine speed, see, for example, U.S. Pat. No. 5,216,915 and U.S. Pat. No. 5,278,760. To equalize the power of the normal torque fluctuations of the internal combustion engine, the internal combustion engine is normally provided with a flywheel which equalizes the speed fluctuations. This implies that the variations which arise in the speed of the internal combustion engine because of misfiring will also decrease. A signal which may be obtained in different ways, and corresponding to these speed variations, will therefore have a low signal level. To this is to be added the fact that the signal is influenced by disturbances from irregularities in the base and a flywheel which is not perfectly balanced. Disturbances may furthermore arise in different operating situations such as during acceleration, changing, disengagement of the clutch etc. The combination of a high speed and a low load is an operational situation which involves problems difficult to handle because periodic inertial forces, generated primarily by the movements of the connecting rods and the pistons, dominate in relation to the force which is developed in and influences the pistons of the cylinders during faultless firing. In summary, the low signal level and the large number of disturbance sources imply that methods based on measurement of the engine speed require calculation-intense methods of evaluation and have a limited reliability.